This invention relates to nuclear reactor fuel assembly grids, and, more particularly, to a bimetallic spacer means operative to resist the occurrence of in-reactor bowing of nuclear reactor fuel assemblies.
The prior art is replete with a multiplicity of teachings of various forms of nuclear fuel assembly grids that have been proposed for use in nuclear reactor cores. One particularly striking aspect of the contents of these prior art teachings is the diversity of structural configurations that have been proposed for embodiment in such fuel assembly grids.
Generally speaking, there are two major functions, which fuel assembly grids are intended to perform. First of all, such grids are used to provide the individual fuel rods, which collectively comprise a fuel assembly, with lateral support. The need for such support is occasioned in large measure by the very nature of the construction of a fuel rod. Namely, each fuel rod commonly consists of a multiplicity of fuel pellets that are arranged in a multiple layer configuration. Moreover, each fuel rod normally is of substantial length. Accordingly, even though the fuel pellets generally are to be found encased in a cladding tube, a need still exists to provide a fuel rod with the aforementioned support to insure that the latter will not undergo lateral displacement to an unacceptable degree, i.e., to a degree which potentially could adversely affect the operation of the nuclear reactor. The susceptiveness of the fuel rods to undergo lateral displacement stems from the nature of the forces to which the fuel rods are subjected. For instance, included in the forces to which reference is had here are the compression spring forces that are being exerted on the ends of the rods, the forces applied to the fuel rods as a consequence of the coolant flow in the fuel assembly, etc.
The other major function, which is performed by the fuel assembly grid, is that of insuring that the desired spacing between the fuel rods, which collectively comprise the fuel assembly, is being maintained. The existence of proper spacing between the individual fuel rods is important, both from the standpoint of insuring that excessive flux peaking occasioned by the existence of improper spacing between fuel rods and the fuel assembly is prevented, and from the standpoint of insuring that proper coolant flow between the fuel rods and the fuel assembly is maintained. Unequal distribution of coolant flow between individual fuel rods can give rise to overheating and eventually cause hot spots to develop within the fuel assembly.
Another important factor which must be borne in mind in proposing for use any particular design for a nuclear fuel assembly is that the support and spacing functions intended to be performed thereby must be capable of being accomplished in such a manner as to not interfere with the process of inserting and removing fuel rods into and from a fuel assembly. Namely, the nuclear fuel assembly grid must be operative to provide the desired lateral support to the individual fuel rods, which collectively comprise the fuel assembly, and to provide the desired spacing therebetween when the fuel rods are emplaced in a fuel assembly, and concomitantly must possess the capability to make possible the facile insertion and/or removal of the fuel rods into and/or from the fuel assembly, when such action is required.
Apart from the need to provide support to the fuel rods themselves, some attention has been directed more recently to the desirability of improving the strength characteristics of the nuclear fuel assembly grid itself. Specifically, reference is had here to the recognition that has been given to the desirability of improving the strength of the fuel assembly grid per se so as to insure the ability of the fuel assembly to withstand the possible subjection thereof to seismic loading. As set forth in U.S. Pat. No. 4,058,436, which issued to the inventor of the subject matter of the instant application and which is assigned to the same assignee as the present invention, such seismic loading could give rise to the subjection of the fuel assembly to severe lateral stresses that, in turn, could adversely affect the operation of the nuclear reactor in which such a fuel assembly is contained. The nature of such adverse effects is clearly outlined in applicant's above-referred to earlier U.S. Patent. It can thus be seen that in proposing a design for a nuclear fuel assembly grid, another consideration which desirably should be taken into account in addition to the need to provide lateral spacing to the fuel rods and to establish the proper spacing between the fuel rods is that of the impact, which seismic loading might have on the fuel assembly.
For the reason set forth in the aforereferenced U.S. Patent, it is important that the strengthening of the fuel assembly grid be accomplished in such a manner as to not detrimentally influence the operating efficiency of the nuclear reactor. That is, it is desirable that the fuel assembly grid be provided with additional rigidity in a manner which will not significantly increase the neutron absorption propensity of the grid. In this regard, as a material zircaloy is known to have a lower neutron capture cross-section than does stainless steel or inconel. On the other hand, as a material stainless steel and inconel are known to possess a higher degree of mechanical strength than does zircaloy.
With the latter in mind, in accord with the teachings of the above-referred to U.S. Patent, a fuel assembly is provided, which has cooperatively associated therewith a multiplicity of all-zircaloy grids and one all-stainless steel grid. Each of the all-zircaloy grids embodies a unique construction that is operative to increase the strength characteristics thereof. Moreover, by employing such all-zircaloy grids, which are characterized by their superior crush strength, in combination with a suitably positioned all-stainless steel grid, a fuel assembly, which is so equipped, is provided that possesses sufficient overall strength to successfully resist the severe lateral stresses that are anticipated under conditions of seismic loading.
In addition to the lateral stresses to which a fuel assembly may be subjected under seismic loading conditions, there are reasons to believe that under certain conditions fuel assemblies equipped with prior art forms of fuel assembly grids, may, for reasons of yet not totally explained, exhibit a susceptiveness to undergo bowing. The term bowing as used herein is intended to refer to that condition of a fuel assembly wherein one or more portions thereof have undergone lateral displacement. Although it has not been caused by seismic loading, such bowing of the fuel assembly is undesired equally as much as the lateral displacement which a fuel assembly undergoes as a consequence of seismic loading. In this regard, the reasons why such bowing of the fuel assembly is undesirable are basically the same as those which are to be found set forth in the U.S. Patent previously referred to hereinabove. Namely, by way of exemplification, such bowing can give rise to the permanent deformation of the fuel assembly. Moreover, the bowing of the fuel assembly may become sufficiently severe as to cause the fuel assembly to impact against adjacent fuel assemblies with adverse consequences. There are other types of damage that are equally likely to occur in the event that such bowing of the fuel assembly takes place with equally detrimental effects on the operation of the nuclear reactor.
Consequently, there has been shown to exist in the prior art a need for a nuclear fuel assembly grid that in addition to possessing the capability of providing lateral support to the fuel rods that collectively comprise the fuel assembly and the capability of effecting the desired spacing between the fuel rods also possesses the capability of providing the fuel assembly with sufficient crush strength to withstand the severe lateral stresses imposed thereupon under seismic loading conditions as well as the ability to resist any susceptiveness on the part of the fuel assembly to undergo bowing. More specifically, a need has been shown to exist for such a grid, which would be operative to prevent the fuel assembly from undergoing lateral displacement that exceeds certain preestablished acceptable limits. Furthermore, a characteristic, which any such grid capable of fulfilling the above-stated objectives must also embody, is the fact that additionally it exhibits a relatively low propensity for neutron absorption.
It is, therefore, an object of the present invention to provide a new and improved bimetallic spacer means particularly suited to be cooperatively associated with a nuclear fuel assembly of the type that is employable in the core of a nuclear reactor.
It is another object of the present invention to provide such a bimetallic spacer means which includes means operative, when the bimetallic spacer means is cooperatively associated with a fuel assembly, to provide lateral support to the fuel rods that collectively comprise the fuel assembly.
It is still another object of the present invention to provide such a bimetallic spacer means which includes means operative, when the bimetallic spacer means is cooperatively associated with a fuel assembly, to provide the proper spacing between the individual fuel rods that collectively comprise the fuel assembly.
A further object of the present invention is to provide such a bimetallic spacer means, which is characterized by its improved crush strength, such that when the bimetallic spacer means is cooperatively associated with a fuel assembly, it is operative to enable the fuel assembly to withstand the severe lateral stresses imparted thereto under seismic loading conditions.
A still further object of the present invention is to provide such a bimetallic spacer means, which is operative when cooperatively associated with a fuel assembly to enable the fuel assembly to resist any susceptiveness thereof to undergo inreactor bowing beyond certain specified preestablished acceptable limits.
Yet another object of the present invention is to provide such a bimetallic spacer means, which is characterized in the fact that it exhibits a relatively low propensity for neutron absorption.
Yet still another object of the present invention is to provide such a bimetallic spacer means, which is relatively inexpensive to manufacture, relatively easy to employ, and which is capable of providing effective and reliable operation.